Broadly, the invention concerns an apparatus useful for cementing a borehole. More specifically, the invention is an improved cementing basket useful for oil well or gas well cementing operations.
When oil wells or gas wells are drilled the usual practice is to cement the annulus between the outside of the pipe casing and the borehole. This is done by using drilling mud to push a cement slurry down through the pipe casing and up into the borehole annulus. When the slurry hardens it provides a cement column in the borehole which has several functions. One job which the cement column does is to prevent corrosive materials in the formation from deteriorating the pipe casing. Another function of the cement column is to keep the borehole wall from sloughing or caving in.
During injection of the cement slurry there is a substantial amount of pressure build up in the borehole annulus. This pressure results from the hydrostatic head created by the slurry column. One of the undesirable effects of the high pressure is cement contamination of the formation at places in the borehole where the formation is weak or porous. The problem can be overcome by cementing the borehole annulus above the weak spot. In this operation the cement basket, which is positioned on the pipe casing, acts as a retainer to hold the wet cement at that spot during the initial set period.
There are various sizes and styles of commercially available cement baskets. A typical basket device is made up of flexible, overlapping petals which will conform to the shape of the borehole. The petals are usually fabricated of flexible materials, such as heavy duty canvas, thin metal, or rubber. A metal backing member, usually a flat spring steel stave, is fastened along one of the lengthwise edges of each petal. A free end of each stave is then welded into a ring structure at the bottom of the basket.
The bottom ring is slightly larger than the pipe casing to permit the basket to slide on the casing. Stop rings are fitted to the casing to hold the basket at a desired position. When the cement slurry rises upwardly through the annulus it pushes around the flexible petals and drops into the basket from the top.
The use of metal staves as backing strips of fabricate the basket petals presents a problem. The most immediate problem is difficulty in obtaining the metal staves, and a secondary problem is the extremely high cost. A particular advantage of the present invention is a cement basket which is easier to fabricate and which utilizes less expensive materials than the baskets described above.